Multifunctionalization of an electronic apparatus has caused a circuit operation on a printed wiring board to be speeded up. In an electric signal flowing through a signal wiring, an operation frequency of a clock signal has been increased, and a rise time/fall time has been shortened. On the other hand, the electronic apparatus has tended to be miniaturized. Therefore, the printed wiring board has been designed to increase a wiring density by making a gap between signal wirings as narrow as possible.
On the background of speeding up the circuit operation and miniaturizing the printed wiring board, crosstalk in which signal waveforms interfere with each other by electromagnetic coupling between adjacent signal wirings on the printed wiring board has been a large issue in recent years. The crosstalk causes a ripple that exceeds a threshold voltage in the adjacent signal wirings to cause an erroneous operation of the apparatus, and becomes, in parallel transmission through memory wirings, jitter of a signal flowing through the adjacent wirings, to cause a timing margin to be consumed.
As means for suppressing such crosstalk, one guard wiring having a stable potential, e.g., one guard ground wiring is arranged between signal wirings. A guard ground wiring is arranged between signal wirings where crosstalk is to be suppressed so that coupling between the signal wirings is reduced, to produce a crosstalk suppression effect.
However, when the crosstalk is required to be further suppressed while there is a design constraint on a wiring area, the crosstalk cannot be suppressed to a desired crosstalk value. Regarding this issue, Japanese Patent Application Laid-Open No. 2005-123520 discusses means for providing two guard ground wirings between signal wirings. In this means, the signal wirings are respectively made to have guard grounds, to further reduce coupling between the signal wirings.
However, the crosstalk tends to be increasing due to the effects of recently speeding up and miniaturizing the electronic apparatus. In the above-described conventional means, a target to reduce the crosstalk may be unattainable. This has required a printed wiring board that produces a further crosstalk suppression effect even if the wiring area between the signal wirings is small.
Therefore, we have considered generation of crosstalk in a conventional printed wiring board. FIG. 5 is a cross-sectional view of the conventional printed wiring board. A printed wiring board 200 includes an insulator layer 201, two signal wirings 202 and 203 arranged on one surface of the insulator layer 201, and two guard ground wirings 204 and 205 arranged between the two signal wirings 202 and 203. A ground plane 206 is arranged on the other surface of the insulator layer 201.
Consider a case where a signal that causes crosstalk to be generated in the signal wiring 202 is transmitted, and the signal wiring 203 receives the crosstalk. Crosstalk noise that propagates to the signal wiring 203 serving as a reference plane includes a direct crosstalk component 211 that directly propagates from the signal wiring 202, and a multiple crosstalk component that propagates from the signal wiring 202 via the guard ground wirings 204 and 205.
The multiple crosstalk component includes a two-stage crosstalk component 212 that propagates from the signal wiring 202 to the guard ground wiring 204 or the guard ground wiring 205, and then propagates from the guard ground wiring 204 or the guard ground wiring 205 to the signal wiring 203. The multiple crosstalk component further includes a three-stage crosstalk component 213 that propagates from the signal wiring 202 to the guard ground wiring 204, then propagates from the guard ground wiring 204 to the guard ground wiring 205, and then propagates from the guard ground wiring 205 to the signal wiring 203.
Generally, in two wirings, a waveform of crosstalk noise generated at an end, far from one of the wirings, of the other wiring is expressed by the following equation (see Circuits, Interconnections, and Packaging for VLSI, H. B. Bakoglu, Andeddison-Wesley Publishing company (1995)).
Math. 1 is as follows.
      V    ⁡          (              l        ,        t            )        =            K      f        ·    l    ·                  ⅆ                  ⅆ          t                    ⁡              [                              V            in                    ⁡                      (                          t              -                              T                d                                      )                          ]            
More specifically, this equation indicates that the crosstalk noise has a waveform obtained by differentiating an input waveform once. Therefore, the direct crosstalk component 211 is considered to have a waveform obtained by differentiating the original signal waveform once. The two-stage crosstalk component 212 is considered to have a waveform obtained by differentiating the original signal waveform two times. Further, the three-stage crosstalk component 213 is considered to have a waveform obtained by differentiating the original signal waveform three times.
Consider a case where a waveform of a signal having a high frequency flowing through the signal wiring 202 is represented by cos θ. The waveform of the direct crosstalk component 211 is represented by −sin θ obtained by differentiating the signal waveform once. The waveform of the two-stage crosstalk component 212 is represented by −cos θ obtained by differentiating the signal waveform cos θ two times. The waveform of the three-stage crosstalk component 213 is represented by +sin θ obtained by differentiating the signal waveform cos θ three times. More specifically, every time crosstalk is generated to propagate between conductors, a phase of a waveform is changed.
At this time, the waveform represented by −sin θ of the direct crosstalk component 211 and the waveform represented by +sin θ of the three-stage crosstalk component 213 cancel each other because they differ in phase by 180 degrees. Therefore, these components are reduced. However, no component does not cancel the waveform represented by −cos θ of the two-stage crosstalk component 212. Therefore, this component finally appears directly as crosstalk noise obtained by synthesis in the signal wiring 203.
More specifically, the guard ground wiring is a wiring, and thus becomes a path of the multiple crosstalk component. In the above-described conventional configuration, the two-stage crosstalk component cannot be reduced. Therefore, the crosstalk noise is difficult to further reduce.